KVM: selftests: Add a VM backpointer to 'struct vcpu'

Add a backpointer to 'struct vcpu' so that tests can get at the owning
VM when passing around a vCPU object.  Long term, this will be little
more than a nice-to-have feature, but in the short term it is a critical
step toward purging the VM+vcpu_id ioctl mess without introducing even
more churn.

Signed-off-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>